Monitoring circuits for direct current circuits

ABSTRACT

The voltage drop across a control resistor in the monitored direct current circuit, which drop varies in accordance with the condition of the direct current load, affects the amount of current through a differential amplifier and causes operation of a transistor switch in one of the two collector circuits of the amplifier whenever the load current changes sufficiently. The switch controls an indicating device.

United States Patent [72] Inventors Gerhard Conzelmann Leinielden; Adolf Kugelmlnn, Leonberg; Heinz Moller, Stuttgart; Heinz Kammerer, Nellingen; Karl-Ernst Boeters, Berlin; Hans-Joachim Fleischer, Berlin, all oi, Germany [2|] Appl. No. 47,516

[22] Filed June 18, 1970 [45] Patented Sept. 14, I971 [73] Assignee Robert Bouh Gmbll Stuttgart, Germany [32] Priority June 18, I969 [33] Germany [54] MONITORING CIRCUITS FOR DIRECT CURRENT [51] Int. Cl l-l03k 5/20 [50] Field of Search 307/235, 297; 315/77, 82; 340/80, 81, 25]

[56] References Cited UNITED STATES PATENTS 3, l 03,6 1 7 9/1963 Schneider et al. 307/297 X 3,428,943 2/1969 Carp et al. 340/80 Primary Examiner-Donald D. Forrer Assistant Examiner-John Zazworsky Attorney-Michael S. Striker ABSTRACT: The voltage drop across a control resistor in the monitored direct current circuit, which drop varies in accordance with the condition of the direct current load, affects CIRCUITS the amount of current throu h g a differential amplifier and 20 Claims 5 Drawing causes operation of a transistor switch in one of the two col- [52] U.S.Cl 307/235, lector circuits of the amplifier whenever the load current 315/77, 340/80, 340/251 changes sufficiently. The switch controls an indicating device.

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. INVENTORS Gerhard CONZELMANN 28 5 30 13 Adolf KU GELMANN wHeinz MULLER Heinz KAMMERER Karl Ernest BOETERS Hans Joachim FLEISCHER their ATTORNEY PATENTED SEP! 4I97l 3,604,949

sum 2 0r 2 FIGA FIGS

T T?" a INVENTORS Gerhard CONZELMANN Adolf KUGELMANN Heinz MOLLER Heinz KAMMERER Kor'l Ernesfi. BOETERS Hans Joachim F'LEISCHER MK M their ATTORNEY MONITORING CIRCUITS FOR DIRECT CURRENT CIRCUITS BACKGROUND OF THE INVENTION The invention relates to the circuitry for monitoringdirect current circuits, particularly in lamp circuits of turn signals on motor vehicles. The monitoring is effected by a potential drop that varies in dependence on the current drawn by the lamps. The potential drop customarily occurs across a control resistor.

It is essential that the driver of the vehicle knows whether the lamp connected in the circuit are blinking when the turn signal is supposed to be operating. For this reason the lamp circuit must be monitored.

The arrangements of the prior art usually use a current relay in the lamp circuit, the relay permitting the lamp to blink only when all lamps are burning. The relay so acts on the blinker unit that the blink frequency is noticeably raised when a lamp fails. This rise in frequency serves as the monitoring system signal that tells that the turn indicator is not operating properly.

Aside from this scheme, employing a current relay, there is also an electronic solution, which obtains a monitoring signal from the voltage drop across a control resistor connected in the lamp circuit.

In order not to affect adversely the warning action of the turn signals in traffic, the vehicle manufacturer and the civil authorities together have stipulated a maximum value that may not be exceeded for the voltage to drop across this resistor. Although in the case of a passenger car usually only two turn signal lamps, both on the same side of the car, operate at the same time, with a truck having a trailer as many as six lamps can blink simultaneously when the direction signals operate. In this instance there also applies the maximum value for the voltage drop across the control resistor.

If costs are to be reduced by not using a switch to shunt the control resistance when the turn signals operate, then only control resistances of low value can be used. Moreover, the blinker unit and its monitoring arrangement must operate reliably over a wide range of temperatures and operating voltages. In addition, the manufacturing tolerances for the electric incandescent lamps and the components of the circuit must be taken into account, as must the fact that in European motor vehicles the negative pole of the battery is connected as a rule to the vehicle frame, so that the control resistor must be connected in the positive line of the lamp circuit.

If all of these demands are to be met, a circuit of discrete components would require a large number of parts, making the circuit expensive. Moreover, because of the spread between characteristic curves, and because of the differences between temperature curves of discrete semiconductor components, extensive and costly adjustments of the circuit are required.

SUMMARY OF THE INVENTION An object of the invention is to provide a monitoring circuit for direct current circuits, particularly for blinker units, such as those of turn signals on motor vehicles, and which monitoring circuit avoids the disadvantages outlined above, particularly the disadvantage of having to adjust the monitoring circuit.

Another object of the invention is a circuit of the preceding object and constructed as a monolithic integrated circuit.

The invention consists fundamentally of a monitored circuit having a load that draws current in dependence on its condition, and two voltage points between which there exists a potential drop that varies in dependence on the current drawn by the load; and a monitoring circuit for producing a signal whenever the load current indicates a predetermined condition of the load, the monitoring circuit including a reference voltage, a differential amplifier having a plurality of transistors, including first and second transistors, a common.

resistance means for the emitters of the first and second transistors, and biasing means, such as for biasing each base of the first and second transistors so that the latter conduct, at least one each of first and second diodes connected with regard to polarities during operation so as to conduct current between the base of a respective one of the first and second transistors and a respective one of the two voltage points, whereby a change in voltage at one of these voltage points atfects the collector current in both of the first and second transistors, means connecting the collector of one of the first and second transistors to that one of the two voltage points which is at the greater potential difference with respect to the reference voltage, and signal generator means including control switch means, which latter are connected between the collector of the other of the first and second transistors and the one voltage point for operation whenever the collector current of the other transistor changes sufficiently in response to a sufficient change in the current through the load.

The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. l is a circuit diagram showing the invention in its fundamental form;

FIG. 2 is a modification of the circuit shown in FIG. 1;

FIG. 3 shows a circuit that can be adjusted for a voltage-dependent load resistance;

FIG. 4 shows the modified part of the circuit of FIG. 3, for obtaining the desired voltage dependence of the switching range; and

FIG. 5 is another embodiment of the invention that is particularly well adapted to manufacture with monolithic integrated circuit techniques.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the circuit of FIG. 1 the positive line 10 contains a (PNP resistance 12 and leads to a load, which is shown as a resistance 11, but in one practical application of the invention can be the filaments of the turn signal lamps. The one end of the load resistance is connected to the minus line 13, which in a motor vehicle is grounded as a rule. This line serves as the reference potential. If opposite types of transistors were employed PNP for NPN and NPN for PNP) then. of course, the polarities of the two lines 10 and 13 would be reversed, but line 13 would still serve as the reference potential. The two voltage points, or ends, A and B of the control resistance 12 are connected to respective diodes l4 and 15, that are also connected by their cathodes to the respective bases of transistors 16 and 17. Since proper polarities are observed, the diodes conduct; and the respective transistor bases are approximately at the voltages at A and B.

The transistors 16 and 17 and their common emitter resistance 18 constitute a differential amplifier. To each base of the transistors 16 and 17 is connected a respective resistance 19 and 20, which set the quiescent current through the diodes l4 and 15. A resistance 21, connected to the collector of transistor 17, shunts the base-emitter path of a complementary transistor 22. The amplified signal is taken off the collector of transistor 22 and conducted to some visual or audible indicating device, such as a signal lamp, to enable monitoring of the turn signals circuit. The transistor 22 together with the indicating device (not shown) constitute a signal generating means.

The monitoring circuit just described operates in the following way. When no current flows through the load ll, the collector currents of the transistors 16 and 17 are equal. The value of the resistance 21 is such that the potential drop across it is sufficient to bias the transistor 22 to conduct. If current begins to pass through the load 11, voltage point B falls with respect to voltage point A, the collector current of transistor 17 declines, and the potential drop across resistance 21 reduces until the transistor 22, which acts as a switch means, is cut off.

This circuit operates satisfactorily provided that the temperature stability and operation over a wide range of voltages are not required. If there is required a high degree of temperature stability, together with a switching range that is independent of the operating voltage, then the circuit of FIG. 2 is particularly advantageous.

In this circuit a resistance 23 in series with a diode 14 shift the switching range of the transistor 22 to the region where the collector currents of the transistors 16 and 17 are equal. Switching occurs when the ratio of resistance 19 to resistance 23 is approximately equal to the ratio of the resistance of load 11 when conducting current to the control resistance 12. To make the switching range in dependent of the operating voltage and to improve the temperature stability, the differential amplifier is fed from a constant current source, which comprises a transistor 24 having as emitter resistance 29 in the common emitter circuit of the transistors 16 and 17. The base of the transistor 24 is connected to the resistance 20 and through the pair of series connected diodes 2S and 26 to the minus line 13. The temperature stability is particularly good if the characteristics of one of these diodes are at least similar to those of the emitter diode of transistor 24 and the characteristics of the other diode are at least similar to those of the emitter diode of transistor 22. Particularly favorable results are obtained if these diodes consist of the emitter diodes of transistors that are of the same type as the respective transistors 22 and 24. The temperature stability of the circuit is further improved if the combined resistance of the parallel connection of the resistance 21 and the direct current input resistance of the transistor 22 is approximately twice the value of the resistance 29. By suitably choosing the working point, the value of the direct current input resistance of the transistor 22 is made advantageously large with respect to the resistance 21.

Although the switching range of the monitoring circuit of FIG. 2 is very largely independent of changes in operating voltage and temperature, it may be necessary, when the load 11 has a voltage-current curve of the general class to which that of a tungsten electric filament belongs, to shift the switching range slightly to accommodate the voltage-dependent resistance of the load 11.

In the circuit of FIG. 3 this kind of voltage-dependent shift is obtained by stabilizing the operating voltage for the differential amplifier and the transistor switch 22. The operating voltage lies between voltage points A and C. The stabilization is ensured by a zener diode 27 connected through a resistance 28 to the minus line 13. The degree of shift can be varied by means of a resistance 30 connected between the line 13 and the base of transistor 16. It may be advantageous, particularly if the circuit is manufactured as a monolithic integrated chip, not to connect the resistance 30 directly to the base but instead to a variable tap on the resistance 19.

FIG. 4 shows another embodiment for adjusting the switching range to the voltage-dependent resistance of the load 11, when the latter, for example, is a turn signal lamp. A resistance 31 is connected in series with the zener diode 27. If the monitoring circuit is to operate reliably over a wide range of voltages, it is possible, in the embodiments shown in FIGS. 3 and 4, that the currents through the resistances 28 and 31 and the zener diode are very large, particularly at the upper voltage limit. The power loss, as a consequence, is great. In monolithic integrated circuits this leads to excessively high temperatures in the crystal, caused by high heat flux densities, and can result in destruction of the circuit.

For this reason in the circuit of FIG. the voltage at point C, intermediate in value between the voltage at point A and the voltage of line 13, is furnished by a current amplifier consisting of a PNP transistor 32 and NPN transistor 33, an expedient that greatly reduces the power loss in this branch. In this embodiment the collector of transistor 22 is connected to the base of a transistor 34, which accomplishes the actual switching. A resistance 35, shunted across the base-emitter path of transistor 34, ensures a particularly narrow switching range of only a few millivolts.

The circuit of FIG. 5 operates in the following manner. Assuming that the load 11 consists of two parallel-connected incandescent electric lamps, the voltage drop across resistance 12 is greater than across resistance 23. The collector current of transistor 17 is therefore smaller than that of transistor 16, and the transistors 22 and 34 are cut off. But if only one lamp flashes, then the voltage drop across resistance 12 is smaller than across resistance 23, the current through transistor 17 is greater than through transistor 16, and both transistors 22 and 34 conduct, causing a signal to be operated. If the signal is to be operated when the transistor 34 is cut off, then transistor 22 and the resistance 21, together with transistor 34 and resistance 35, are connected into the collector circuit of transistor 16.

The series-connected resistance 23 and diode l4, explained in description of the embodiment of FIG. 2, can also be used in the embodiment of FIG. 1. The additional features, described in connection with the embodiments of FIGS. 3, 4 and 5, can also be incorporated into the simple circuit of FIG. 1.

If the monitoring circuit of the invention is constructed as a monolithic integrated circuit, components that mutually affect the electrical performance of each other if their temperatures change are advantageously located, in so far as possible, on isotherms of the semiconductor chip. Numbered among these components are, for example, the transistors 16 and 17; the diode pair 14 and 15; the transistor 24 with the diode 25; the diode 26 and the transistor 22; the resistances 19, 20 and 23; or 28 and 31; or 29 and 21. In the case of the diodes 25 and 26, it has been assumed that the characteristics of the former correspond to those of the emitter diode of transistor 24, and those of the latter to the characteristics of the emitter diode of transistor 22.

If the circuit (the monitored circuit) for the turn signals is constructed on a monolithic crystal, or chip, the monitoring circuit of the invention is advantageously incorporated on the same chip.

It will be understood that each of the elements described above, or two or more together, may also find a useful application in other types of circuits differing from the types described above.

While the invention has been illustrated and described as embodied in monitoring circuits for direct current circuits, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can by applying current knowledge readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitutes essential characteristics of the generic or specific aspects of this invention and, therefore such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims.

1. In a circuit of the kind described, a monitored circuit having load means that draws current in dependence on its condition, and two voltage points between which there exists a potential drop that varies in dependence on the current drawn by said load means; and a monitoring circuit for producing a signal whenever the load current indicates a predetermined condition of said load, said monitoring circuit including a reference potential; a differential amplifier having a plurality of transistors, including first and second transistors, common resistance means for the emitters of said first and second transistors, and biasing means for biasing each base of said first and second transistors so that the latter conduct; at least one each of first and second diodes connected with regard to polarities during operation so as to conduct current between the base of a respective one of said first and second transistors and a respective one of said two voltage points, whereby a change in voltage at one of said voltage points affects the collector current in both of said first and second transistors; means connecting the collector of one of said first and second transistors to that one of said two voltage points which is at the greater potential difference with respect to said reference potential; and signal generating means including control switch means, said control switch means being connected between the collector of the other of said first and second transistors and said one voltage point for operation whenever the collector current of said other transistor changes sufficiently in response to a sufficient change in the current through said load means.

2. A monitoring circuit as defined in claim 1, wherein said biasing means are first resistance means connected in the base of said one transistor and second resistance means connected in the base of said other transistor.

3. A monitoring circuit as defined in claim 2, wherein said switch means comprise a transistor and third resistance means for biasing and connected between the base and the emitter of the transistor of said switch means, the latter transistor being complementary to said other transistor and connected between the collector of the latter and said one voltage point so that its conductive state depends on the amount of the collector current of said other transistor.

4. A monitoring circuit as defined in claim 3, including a further transistor of which the base is connected to the collector of said switch means transistor, and resistance means connected between the emitter and the base of said further transistor, whereby the conductive state of said further transistor depends on the conductive state of said switch means transistor.

5. A monitoring circuit as defined in claim 2, including a constant current source connected in the emitters of said first and second transistors.

6. A monitoring circuit as defined in claim 5, wherein said constant current source includes at least one third transistor of which the collector is connected to the emitters of said first and second transistors; at least one diode conductively connecting the base of said third transistor to a voltage of correct polarity so that said third transistor is biased to conduct; and wherein said common resistance means are connected between said voltage of correct polarity and the emitter of said third transistor.

7. A monitoring circuit as defined in claim 2, including control resistance means connected in said monitored circuit and across which said potential drop occurs.

8. A monitoring circuit as defined in claim 2, wherein third resistance means are connected in series with that one of said first and second diodes connected to said one voltage point at the greater potential difference.

9. A monitoring circuit as defined in claim 8, including a control resistance means connected in said monitored circuit and across which said potential drop occurs, and wherein the desired switching range of said switch means occurs where the resistance ratio of said third resistance means to said first resistance means is approximately equal to that of said control resistance means to said load means under current.

10. A monitoring circuit as defined in claim 9, wherein said switch means comprise a transistor and third resistance means for biasing and connected between the base and the emitter of the transistor of said switch means, the latter transistor being complementary to said other transistor and connected between the collector of the latter and said one voltage point so that is conductive state depends on the amount of the collector current of said other transistor and so that its direct current input resistance occurs between its base and emitter; and

wherein in the emitter of said other transistor of said first and second transistors the resistance resulting from the parallel connection between said third resistance means and the direct current input resistance of the transistor of said switch means has approximately twice the value of said common resistance means.

11. A monitoring circuit as defined in claim 10, including a constant current source connected in the emitter of said first and second transistors, said constant current source comprising at least one third transistor of which the collector is connected to the emitter of said first and second transistors; and third and fourth diodes in series conductively connecting the base of said third transistor to a voltage of correct polarity so that said third transistor is biased to conduct; and wherein one of said third and fourth diodes consists of the emitter diode of a transistor of the same type as the transistor of said switch means and the other of said third and fourth diodes consists of the emitter diode of a transistor of the same type as said third transistor so as to ensure that the current flowing through said third and fourth diodes is approximately equal to the current flowing through the transistor of said switch means and said third transistor.

12. A monitoring circuit as defined in claim 11, wherein said load means exhibits a voltage-dependent resistance, and including means for shifting the switching range in dependence on said voltage-dependent resistance, said means including voltage stabilization means for stabilizing the operating voltage for said monitor circuit.

13. A monitoring circuit as defined in claim 12, wherein said voltage stabilization means include a series connected zener diode and fourth resistance means connected between said reference potential and said one voltage point.

14. A monitoring circuit as defined in claim 13, including means for adjusting the degree to which the switching range is shifted, said means including a voltage intermediate said reference potential and said one voltage point, and wherein one of said first and second resistance means is connected to said intermediate voltage and has a variable tap, and further resistance means connected between said tap and said reference potential.

15. A monitoring circuit as defined in claim 13, including means for adjusting the degree to which the switching range is shifted, said means comprising fifth resistance means connected in series with said zener diode.

16. A monitoring circuit as defined in claim 15, wherein the series connection of said zener diode and said fourth and fifth resistance means comprises a voltage intermediate said reference potential and said one voltage point, and wherein that one of said first and second resistance means in the base of said one transistor is connected to said intermediate voltage.

17. A monitoring circuit as defined in claim 16, including a current amplifier connected to furnish said intermediate voltage.

18. A monitoring circuit as defined in claim 17, wherein said current amplifier comprises a pair of complementary transistors.

19. A monitoring circuit as defined in claim 1, wherein said monitoring circuit is a monolithic integrated circuit and mutually affecting temperature sensitive components thereof are placed along at least approximate isotherms of the semiconductor crystal so as to reduce temperature differences between these components.

20. A monitoring circuit as defined in claim 1, wherein all components of said monitored circuit and said monitoring circuit that can be made part of an integrated circuit are combined in a monolithic circuit on a common semiconductor crystal. 

1. In a circuit of the kind described, a monitored circuit having load means that draws current in dependence on its condition, and two voltage points between which there exists a potential drop that varies in dependence on the current drawn by said load means; and a monitoring circuit for producing a signal whenever the load current indicates a predetermined condition of said load, said monitoring circuit including a reference potential; a differential amplifier having a plurality of transistors, including first and second transistors, common resistance means for the emitters of said first and second transistors, and biasing means for biasing each base of said first and second transistors so that the latter conduct; at least one each of first and second diodes connected with regard to polarities during operation so as to conduct current between the base of a respective one of said first and second transistors and a respective one of said two voltage points, whereby a change in voltage at one of said voltage points affects the collector current in both of said first and second transistors; means connecting the collector of one of said first and second transistors to that one of said two voltage points which is at the greater potential difference with respect to said reference potential; and signal generating means including control switch means, said control switch means being connected between the collector of the other of said first and second transistors and said one voltage point for operation whenever the collector current of said other transistor changes sufficiently in response to a sufficient change in the current through said load means.
 2. A monitoring circuit as defined in claim 1, wherein said biasing means are first resistance means connected in the base of said one transistor and second resistance means connected in the base of said other transistor.
 3. A monitoring circuit as defined in claim 2, wherein said switch means comprise a transistor and third resistance means for biasing and connected between the base and the emitter of the transistor of said switch means, the latter transistor being complementary to said other transistor and connected between the collector of the latter and said one voltage point so that its conductive state depends on the amount of the collector current of said other transistor.
 4. A monitoring circuit as defined in claim 3, including a further transistor of which the base is connected to the collector of said switch means transistor, and resistance means connected between the emitter and the base of said further transistor, whereby the conductive state of said further transistor depends on the conductive state of said switch means transistor.
 5. A monitoring circuit as defined in claim 2, including a constant current source connected in the emitters of said first and second transistors.
 6. A monitoring circuit as defined in claim 5, wherein said constant current source includes at least one third transistor of which the collector is connected to the emitters of said fIrst and second transistors; at least one diode conductively connecting the base of said third transistor to a voltage of correct polarity so that said third transistor is biased to conduct; and wherein said common resistance means are connected between said voltage of correct polarity and the emitter of said third transistor.
 7. A monitoring circuit as defined in claim 2, including control resistance means connected in said monitored circuit and across which said potential drop occurs.
 8. A monitoring circuit as defined in claim 2, wherein third resistance means are connected in series with that one of said first and second diodes connected to said one voltage point at the greater potential difference.
 9. A monitoring circuit as defined in claim 8, including a control resistance means connected in said monitored circuit and across which said potential drop occurs, and wherein the desired switching range of said switch means occurs where the resistance ratio of said third resistance means to said first resistance means is approximately equal to that of said control resistance means to said load means under current.
 10. A monitoring circuit as defined in claim 9, wherein said switch means comprise a transistor and third resistance means for biasing and connected between the base and the emitter of the transistor of said switch means, the latter transistor being complementary to said other transistor and connected between the collector of the latter and said one voltage point so that is conductive state depends on the amount of the collector current of said other transistor and so that its direct current input resistance occurs between its base and emitter; and wherein in the emitter of said other transistor of said first and second transistors the resistance resulting from the parallel connection between said third resistance means and the direct current input resistance of the transistor of said switch means has approximately twice the value of said common resistance means.
 11. A monitoring circuit as defined in claim 10, including a constant current source connected in the emitter of said first and second transistors, said constant current source comprising at least one third transistor of which the collector is connected to the emitter of said first and second transistors; and third and fourth diodes in series conductively connecting the base of said third transistor to a voltage of correct polarity so that said third transistor is biased to conduct; and wherein one of said third and fourth diodes consists of the emitter diode of a transistor of the same type as the transistor of said switch means and the other of said third and fourth diodes consists of the emitter diode of a transistor of the same type as said third transistor so as to ensure that the current flowing through said third and fourth diodes is approximately equal to the current flowing through the transistor of said switch means and said third transistor.
 12. A monitoring circuit as defined in claim 11, wherein said load means exhibits a voltage-dependent resistance, and including means for shifting the switching range in dependence on said voltage-dependent resistance, said means including voltage stabilization means for stabilizing the operating voltage for said monitor circuit.
 13. A monitoring circuit as defined in claim 12, wherein said voltage stabilization means include a series connected zener diode and fourth resistance means connected between said reference potential and said one voltage point.
 14. A monitoring circuit as defined in claim 13, including means for adjusting the degree to which the switching range is shifted, said means including a voltage intermediate said reference potential and said one voltage point, and wherein one of said first and second resistance means is connected to said intermediate voltage and has a variable tap, and further resistance means connected between said tap and said reference potential.
 15. A monitoring circuit as defineD in claim 13, including means for adjusting the degree to which the switching range is shifted, said means comprising fifth resistance means connected in series with said zener diode.
 16. A monitoring circuit as defined in claim 15, wherein the series connection of said zener diode and said fourth and fifth resistance means comprises a voltage intermediate said reference potential and said one voltage point, and wherein that one of said first and second resistance means in the base of said one transistor is connected to said intermediate voltage.
 17. A monitoring circuit as defined in claim 16, including a current amplifier connected to furnish said intermediate voltage.
 18. A monitoring circuit as defined in claim 17, wherein said current amplifier comprises a pair of complementary transistors.
 19. A monitoring circuit as defined in claim 1, wherein said monitoring circuit is a monolithic integrated circuit and mutually affecting temperature sensitive components thereof are placed along at least approximate isotherms of the semiconductor crystal so as to reduce temperature differences between these components.
 20. A monitoring circuit as defined in claim 1, wherein all components of said monitored circuit and said monitoring circuit that can be made part of an integrated circuit are combined in a monolithic circuit on a common semiconductor crystal. 